1. Field of the Disclosure
The present disclosure relates to a bandwidth allocation method for dynamically allocating an upstream communication time (bandwidth) in a passive optical network and a passive optical network system in which the bandwidth allocation method is adopted.
2. Discussion of the Background Art
Recently users of FTTH (Fiber To The Home) services are increasing with development of the Internet. A PON (Passive Optical Network) including an OLT (Optical Line Terminal) placed in a central office of a communication carrier and an ONU (Optical Network Unit) placed in a user's home is one of modes of FTTH.
The PON is an optical network in which the OLT and the plural ONUs are connected through an optical transmission line such as an optical fiber and a passive optical splitter, and the PON is widely used because of its economical superiority.
In an EPON (Ethernet (a registered trademark) PON) in which the OLT and the ONU conduct communication through an Ethernet (a registered trademark) frame, because a GE-PON (Gigabit Ethernet (a registered trademark) PON) having a transmission speed of 1 Gbps can provide the high-rate, and inexpensive FTTH service, in particular the GE-PON is widely used within the country. Recently there is a studied of 10 G-EPON whose transmission speed is enhanced to 10 Gbps.
Generally, in the PON, a communication direction from the OLT to the ONU is called a downstream direction, and the opposite direction to the downstream direction is called an upstream direction. In many PONs including the GE-PON, the communication in the upstream direction is performed by time-division multiple access. The OLT controls transmission timing of each ONU, which allows the plural ONUs to conduct communication with the OLT in the time-division manner.
Similarly the upstream communication of the 10 G-EPON is conducted by the time-division multiple access. In the 10 G-EPON, there is a studied method in which the plural ONUs having the different upstream transmission speeds can be connected to one OLT. At this point, the upstream communication is realized by the time-division multiple access even between the OLT and the ONUs having the different upstream transmission speeds.
In many PONs in which the communication in the upstream direction is conducted by the time-division multiple access, in order to efficiently utilize the upstream bandwidth, dynamically allocated bandwidth is required by dynamically changing a time length in which the upstream communication of each ONU is permitted according to a communication state. In this case, a transmission permission amount is calculated in each ONU, and its transmission time band is exclusively secured, thereby allocating the bandwidth.
In the GE-PON, a protocol called MPCP (Multi Point-Control Protocol) is defined in order to control transmission timing of each of the plural ONUs. The OLT performs the dynamic allocated bandwidth using the MPCP.
The ONU transmits a transmission request signal to the OLT while a transmission waiting upstream data amount is described as a transmission waiting amount in the transmission request signal. The OLT receives the transmission request signal, refers to a transmission requirement, calculates a transmission permission amount and a transmission starting time in each ONU, and transmits a transmission permission signal to each ONU while the transmission permission amount and the transmission starting time are described in the transmission permission signal. At this point, the OLT performs control such that the ONU transmission signals do not temporally overlap one another. The ONU follows the received transmission permission signal to start the transmission from the specified transmission starting time, and transmits the signal accumulated in an upstream buffer only for the time of the specified transmission permission amount.
The dynamic bandwidth allocation method is important in determining performance of the PON system. The performance of the PON system can be evaluated using indexes such as bandwidth utilization efficiency and a speed of the convergence of the actual allocated bandwidth on the target bandwidth. In the dynamic allocated bandwidth, it is necessary that the target bandwidth be calculated from the communication state of each ONU while the high bandwidth utilization efficiency is maintained, and that the actual allocated bandwidth be brought close to the target bandwidth. At this point, the speed of the convergence of the actual upstream allocated bandwidth on the target bandwidth is largely influenced by the dynamic bandwidth allocation method.
It can be said that it is a high performance PON system that the average allocation bandwidth of actual time converges faster to the target bandwidth.
Therefore, in one of the dynamic bandwidth allocation methods, the OLT provides a requirement threshold that is of a transmission requirement threshold to the ONU, causes the ONU to report a buffer accumulation amount that is equal to or lower than the threshold as a transmission requirement to the OLT, and notifies the ONU of the transmission permission amount matched with the transmission requirement that is less than equal to the threshold reported from the ONU.
Japanese Unexamined Patent Application Publication No. 2004-528740 discloses a method for controlling the upstream allocated bandwidth by providing an upper limit in the upstream transmission requirement of each ONU. In Japanese Unexamined Patent Application Publication No. 2004-528740, each ONU notifies the OLT of the transmission requirement, and the OLT permits the transmission of the amount that is matched with the notified transmission requirement, so that a surplus bandwidth is eliminated and the bandwidth utilization efficiency is improved. At this point, the upper limit is set to the transmission requirement of each ONU in order to prevent the state in which the bandwidth is exclusively allocated to the ONU having the traffic amount larger than other ONUs and to realize SLA (Service Level Agreement). The allocated bandwidth can be controlled by setting the upper limit.
However, in the conventional method, there are problems that the upper limit of the transmission requirement is not suitable to the quick convergence of the allocated bandwidth on the target bandwidth, and the convergence of the allocated bandwidth on the target bandwidth cannot be controlled.
That is, in Japanese Unexamined Patent Application Publication No. 2004-528740, the allocated bandwidth is controlled by setting the upper limit of the transmission requirement in order to avoid the bandwidth monopolization of the particular ONU and to realize the SLA. Therefore, because the upper limit of the transmission requirement is fixed to a value that is not suitable to the control of the allocated bandwidth, not only the allocated bandwidth cannot converge quickly on the target bandwidth, but also the way of the convergence cannot be controlled. In Japanese Unexamined Patent Application Publication No. 2004-528740, in addition to the above-described method, there is also proposed a method in which the upper limit of the transmission requirement is dynamically adjusted based on an input traffic to the ONU from a connected terminal or the like. However, there is disclosed no specific procedure of adjusting the upper limit of the transmission requirement.
An object of the disclosure is to provide a bandwidth allocation method in which, when the PON upstream bandwidth is dynamically allocated, the way of convergence can be controlled while the time average allocated bandwidth converges quickly on the target bandwidth, and a passive optical network system in which the bandwidth allocation method is adopted.